Direct Sequence Spread Spectrum (DSSS) is an encoding technique used in wireless communications systems. DSSS wireless systems typically transmit data by encoding “1”s and “0”s as different Pseudo-Noise (PN) codes. An exemplary DSSS encoding scheme is shown in FIG. 1. The PN codes corresponding to a ‘1’ value and a ‘00’ value are shown and are complimentary. Several different DSSS demodulation techniques are used today. In one exemplary demodulation technique, the received signal is decoded by passing it through a digital correlator which converts the received pseudo-noise signal back into data bits.
The DSSS systems exhibit longer range and better interference immunity than conventional Frequency Shift Key (FSK) radio systems because the correlator allows correct decoding of corrupted PN code portions. However, these advantages come at the expense of reduced throughput, as the data rate is equal to the length of the PN code and is only a fraction of the raw modulation frequency.
In one example, a transceiver device encodes transmit data as either 32 or 64 “chip” PN codes. One chip is one data element transmitted at the raw modulation frequency of the radio. In most DSSS radios, a data “1” bit is transmitted as the PN code and a data “0” bit is transmitted as the inverse of that same PN code. The receiver comprises a digital correlator that decodes a high correlation (i.e. where most of the chips match) as a “1” value and decodes a low correlation (i.e. where most of the chips do not match) as a “0” value.
FIG. 2 shows an exemplary transmitted DSSS encoding. Firmware running on a microcontroller (MCU) coupled with a radio sends an 24-byte data packet 0x275493 by loading 0b00100111 into a Transmit (TX) data register and 0b1111111 into a data valid register, followed by 01010100 and 0b11111111, and then 0b10010011 and 0b11111111, respectively. This transmission takes 24 bit periods. The data valid register is used to gate transmission of the data in the TX data register, and these bits have a 1-1 correspondence.
Decoding may be configured so that when using 64-chip PN codes a correlation of 56 or more chips within the 64 bit code is decoded as a “1” and a correlation of 8 or fewer chips is decoded as a “0”. If the correlation result is between 9 and 55 no data is decoded. Interference or a high signal-noise ratio may prevent some bits in a packet from being decoded. This is called an ‘erasure’. When no data is being transmitted, generally no data will be decoded. Although, random noise may cause occasional spurious correlations.
Data is transmitted in packets in some wireless systems and the end of a packet is typically detected after three successive non-correlations. In other words, an end of packet is detected when no 1 or 0 value is successfully decoded for three successive correlation periods. Conventional DSSS systems can therefore detect 3 states, “1”, “0” and “E” (erasure).
As mentioned above, one disadvantage of conventional DSSS radio systems is that the throughput/bit-rate is significantly less than the raw modulation frequency. It would be desirable to increase data throughput for spread spectrum systems.